1. A device was developed for the measurements of the size distribution of radioactive ions. The device was consisted of following three parts.1) Mobility analyzerFor measuring the mobility distribution of ions, a so-called second order differential method was adopted. The analyzer developed was consisted of a cylindrical condenser of the type of Gerdien aspirator and a radioactive aerosol collector provided with a silicon semiconductor detectors. Dimensions of the analyzer and air flow rates are as follows ; Diameter of the outer electrode : 12 cm, Diameter of the inner electrode : 10 cm, Length of inner electrode : first segment 102 cm and second segment 55 cm, Sample air flow rate : 12.5 1/min and Clean air flow rate : 27.5 1/min.2) D.C sourceFor scanning the mobility range * 0.5-10 x 10^<-4>cm^2/v.sec, a D.C source which consists of a stabilizer, a rectifier and a potential divider was developed.3) A device for the collection and analysis of the radioactive aerosolsA device consisted of roll filter, two silicon semiconductor detectors, and two pulse hight analyzers was developed. By this device, simultaneous measurements of growth and decay of the radioactive aerosols collected on the filter are possible.2. At the present time, it leaves something to be desired, but the device developed and newly constructed is thought to be useful to measure the size distribution of radioactive ions with fairly accurate resolution and measuring accuracy.